In the present disclosure reference is mainly made to the treatment of diabetes by injection of insulin. However, this is merely an exemplary use of the present invention. Thus, the present invention may be used for injection of any other suitable kind of drug, e.g. growth hormone.
Injection devices, e.g. in the form of injection pens, are mainly made for users who have to inject themselves frequently, e.g. people having insulin-dependent diabetes or needing treatment by growth hormones. A number of requirements are set to such injection devices. The setting of a dose must be easy and unambiguous and it must be easy to read the set dose. Furthermore, it must be possible, with a minimum of trouble, to cancel or change a wrongly set dose. Finally, when the dose is injected the dose setting mechanism must return to zero. This is very important since it ensures that the set dose is actually injected, thereby allowing the user to keep track of which dose is injected.
Many injection devices work with a threaded piston rod which cooperates with a nut, the nut and the piston being capable of rotating relatively to each other. The dose setting may be obtained by dialling the nut away from a stop to which it is returned during injection by pressing the piston rod forward, either manually or by means of a mechanically biased mechanism, such as a spring, until the nut member abuts the stop. In other injection devices one of the elements, the nut or the piston rod, is kept inrotatable while the other one is allowed to rotate a set angle depending on the set dose, whereby the piston rod is dialled a distance in a forward direction through the nut member.
In such prior art injection devices a dose is normally set by dialling a dose setting member, and the set dose is injected by pushing an injection button. In elongated pen shaped injection devices the dose setting member and the injection button normally form a single member. When the injection button is pushed the set dose is expelled. However, the amount of drug expelled is only equal to the set dose if the injection button has been pushed as far as possible, the dose setting member thereby having been brought back to zero. In order to ensure that the correct dose has actually been injected, the user therefore has to visually inspect the position of the dose setting member during the injection. This is disadvantageous because the injection in some cases will take place in a part of the body where visual inspection during the injection is very difficult or even impossible. Furthermore, in case the user is visually impaired it may be difficult for the user to visually inspect the dose setting member during or after the injection, regardless of where on the body the injection is performed. Since it is not uncommon for people having diabetes to be visually impaired, this is an important aspect.
It is therefore desirable to provide a feedback signal to the user indicating that the set dose has been injected, the feedback signal being of a kind which makes it unnecessary for the user to visually inspect whether or not the set dose is injected.
Some prior art injection devices have a mechanism which informs the user that a dose is being injected by producing an audible ‘click’ for each dose unit being injected. However, since these clicks appear during the entire injection they do not provide a feedback signal indicating that the set dose has been injected, and the problem indicated above is therefore not solved by these injection devices. Prior art injection devices of this type are, e.g., described in U.S. Pat. No. 4,592,745, EP 0 688 571 and US 2004/0210199.
In WO 98/57688 an injection device is disclosed which addresses the above mentioned problem. Thus, WO 98/57688 discloses an injection device having a dose setting device. A dose is set by dialling a dose setting member. Apart from setting a dose the dialling action causes an injection button to be moved from a position where it abuts a housing of the injection device to a position where it protrudes from the housing. The set dose is subsequently delivered by pushing the injection button back into abutment with the housing.
In one embodiment a lock is activated when the injection button reaches the housing, and the activation of the lock produces an audible click indicating that the injection button is in abutment with the housing and thereby that the set dose has been delivered. During the injection, including the final part when the lock is activated, the injection button is moved linearly. The linear distance travelled by the injection button during the last few doses is relatively short. It may therefore be difficult to determine accurately from the audible click produced by the lock whether or not and when the set dose has been delivered.
EP 0 594 357 discloses another injection device which addresses the above mentioned problem. Thus, EP 0 594 357 discloses an injection device having a top section with resilient legs depending perpendicularly from the top section. The outer surface of the resilient legs has a ridge which rests on a ledge inside of the dose knob. The dose knob may have an elongated section which fits into a cylindrical sleeve such that when the dose knob is pushed into the sleeve, at the end of injection, the top portion of the sleeve touches end of the leg of the resilient legs displacing the ridge from the ledge and causing a snapping noise. As it is the case with the injection device described in WO 98/57688, the dose knob is moved linearly during injection, also during the final part of the injection when the resilient legs are displaced from the ridge causing the snapping noise. Therefore the shortcomings described above are also applicable here.